Interleukin 5 (IL-5) and Interleukin 3 (IL-3) mRNA levels in human peripheral blood T cells were compared by semi-quantitative polymerase chain reaction (PCR) analysis. Unstimulated T Cells did not express IL-5 and IL-3 mRNA. IL-5 and IL-3 mRNA expression were similarly induced by the lectin concanavalin A (Con A). The protein kinase C (PKC) activator phorbol myristate acetate (PMA) triggered both IL-3 and IL-5 mRNA expression, whereby IL-5 and IL-3 mRNA expression was observed after 9 and 3 h treatment, respectively. Stimulation with calcium ionophore A23187 induced IL-3 mRNA expression, whereas it failed to induce IL-5 mRNA. In contrast to IL-3 mRNA, the expression of IL-5 mRNA was dependent on de novo protein synthesis, since cycloheximide (CHX) blocked the Con A plus PMA induced IL-5 mRNA expression. In contrast, cyclosporin A (CsA) inhibited but failed to completely block the expression of IL-3 and IL-5 mRNA. mRNA studies in T cell subsets revealed that the expression of IL-5 mRNA was restricted to the CD4 positive T cell subset in response to Con A plus PMA stimulation. On the other hand, IL-3 mRNA expression was noticed in both the CD4 and the CD8 positive T cell subset. These data indicate that the selective expression of IL-5 by human T cells can either be explained by activation of a selective intracellular signalling pathway or by selective activation of a T cell subset. Alternatively, both processes could be involved.